FIG. 12 is a basic circuit diagram of dual transformer type DC-DC converter disclosed in Japanese Unexamined Patent Publication No. 2005-51995. A first transformer T101 includes primary windings W101, W102, and a secondary winding W103, and a second transformer T102 includes primary windings W104, W105, and a secondary winding W106. In a period of discharging a capacitor C101 by turning on a main switch Q101, and turning off a sub switch Q102, energy is transmitted from the primary windings W105 and W104 to the secondary winding W106 side, and the primary windings W101, W102 accumulate magnetic energy. On the other hand, in a period of charging the capacitor C101 by turning off the main switch Q101, and turning on the sub switch Q102, energy is transmitted from the primary windings W101 and W102 to the secondary winding W103 side, and the primary windings W104, W105 accumulate magnetic energy.
FIG. 13 shows a structure of a transformer core integrating two transformers used in the DC-DC converter shown in FIG. 12. An I-shaped core 3000 is disposed on an E-shaped core 2000 and a first side wall 2003, and a gap G101 is formed between the I-shaped core 3000 and a center column 2002. Accordingly, a first gap closing magnetic circuit 6000 is formed to pass through the I-shaped core 3000, the first side wall 2003, a bottom plate 2001, the center column 2002, the gap G101, and the I-shaped core 3000. The gap closing magnetic circuit 6000 is a magnetic circuit for the first transformer T101. An I-shaped core 4000 is disposed on the E-shaped core 2000 and a second side wall 2004, and a gap G102 is formed between the I-shaped core 4000 and the center column 2002. Accordingly, a second gap closing magnetic circuit 7000 is formed to pass through the I-shaped core 4000, the second side wall 2004, the bottom plate 2001, the center column 2002, the gap G102, and the I-shaped core 4000. The gap closing magnetic circuit 7000 is a magnetic circuit for the second transformer T102.
Primary windings W101, W104 are formed integrally, and wound around the center column 2002 by a specified number of turns, and similarly primary windings W102, W105 are formed integrally, and wound around the center column 2002 by a specified number of turns. Secondary windings W103, W106 are wound on the center column 2002 reversely by a half turn. Thus, a common transformer is formed by integrating first and second transformers T101, T102. As other related techniques, there are known the DC-DC converters disclosed in Japanese Unexamined Patent Publication Nos. 2005-51994, 2003-79142, 2002-57045, and 2000-353627.